1. Field of the Invention
The present invention relates to a mechanical pencil in which lead feed is done automatically upon release of writing of a slider from the paper surface or the like and the slider can be locked in a retreated position when the pencil is not in use.
2. Description of the Prior Art
Recently there have been proposed automatic mechanical pencils containing a known one-way type chuck unit and capable of effecting rear-end knock, in which a chuck grips lead upon application of writing pressure, while the gripping action of the chuck for the lead is released upon release of the writing pressure, and by a biasing force of a slider acting toward the front end side for writing and by a friction imparting portion inserted in the slider, the lead itself leaves the chuck toward the front end side and is thereby fed for writing.
In all of such proposals, however, the internal structure such as a lead feed mechanism is very complicated, the assembly work efficiency is poor and the number of components is large.
In view of the above problems the applicant in the present case has previously proposed a mechanical pencil in Japanese patent application No. 298641/85 as an original structure of the aforesaid type of mechanical pencils.
According to this proposed mechanical pencil, lead feed can be effected in three ways (automatic writing, front-end knock and rear-end knock) under a relatively simple internal structure. In the lead feed operation by front-end knock, the lead is fed, of course, when it is not projecting from the front end, while when the lead is projecting in excess of a predetermined amount, it is not fed any further, with only cushioning being performed.
However, the foregoing known automatic mechanical pencils and the mechanical pencil of the above prior application are of a structure in which the slider moves alone and the lead is fed out by such movement. Therefore, in carrying the pencil after use, for example when the pencil is put into a pocket, the slider may retreat unnecessarily, allowing only the lead to be fed out inadvertently, thus causing stain of the clothing or breakage of the lead.
In the prior art, moreover, there is a predetermined space between the lead feed mechanism and the slider, with no lead protecting means provided therebetween, so there has been the problem that the lead is broken by some external force and the broken pieces get into the lead feed mechanism and are caught in the lead chuck, causing trouble. Moreover, an extremely complicated internal structure is required for unlocking and locking and stowing the slider, resulting in that the assembling work becomes less efficient and the number of components increases inevitably.
Further, a conventional lead stopper mounted in the interior of the front end side of a mechanical pencil to exert a predetermined frictional force on the lead is constituted by a friction imparting member provided in the interior of a tip member of the mechanical pencil or in the interior of a slider which is slidable axially in the interior of the tip member. The friction imparting member is integrally formed in the shape of a stepped cylinder comprising a cylindrical portion of a large diameter located on the front end side and a cylindrical portion of a large diameter on the rear end side, with a lead insertion hole being formed through the axes of those cylindrical portions.
The friction imparting member is mounted by press-fitting into the tip member or the slider with its small-diameter cylindrical portion facing forward, whereby the entire outer peripheral surface of the friction imparting member is held in frictional engagement with the inner peripheral surface of the tip member or the slider.
In this state, if the lead is inserted into the lead insertion hole of the friction imparting member, a predetermined frictional force is imparted to the lead by the inner peripheral surface of the lead insertion hole. Drop-out of the lead is thereby prevented during writing and the lead is provided with stable support and a smooth lead feed operation.
However, in the conventional lead stopper in an assembled state, the entire outer peripheral surface of the friction imparting member comes into frictional engagement with the inner peripheral surface of the tip member or the slider without leaving any gap, so variations in the radial direction of the friction imparting member and the tip member or the slider as well as variations in the lead diameter cannot be absorbed because it is impossible for the friction imparting member to undergo elastic deformation in a diameter expanding direction.
For example, if the outside diameter of the lead is 0.58 mm, the inside diameter of the lead insertion hole of the friction imparting member is 0.53 mm, the outside diameter of the lead insertion hole is 1.87 mm and the inside diameter of the slider is 1.77 mm, and when the lead is inserted into the lead insertion hole, there occurs in the friction imparting member an expansion of 0.05 mm in relation to the lead diameter and an expansion of 0.07 mm in relation to the slider, that is, an expansion of 0.12 mm in total. But this expansion cannot be absorbed by the friction imparting member which is incapable of undergoing elastic deformation in the diameter expanding direction. Consequently, it becomes no longer possible to impart an appropriate frictional force to the lead and so it becomes impossible to prevent drop-out of the lead and attain stable support and smooth lead feed operation.
Further, since the friction imparting member is formed in the shape of a stepped cylinder and has directionality at the time of mounting, etc., its mounting work is less efficient and the molding die required becomes complicated in structure, thus leading to increase of the cost.
Additionally, the conventional lead chuck is an integral chuck having a slot for opening. But recently there has been developed a lead chuck which per se is divided completely in plural portions.
There has been a lead chuck of such one-way type in which the lead opening operation is performed by pressing the rear end of the lead chuck in a picking form and by the utilization of a lever action using a part of the lead chuck as a fulcrum. However, such a way of lead opening operation causes deviation at the front end of the lead chuck, so when the chuck which is divided in plural portions returns to the lead gripping position, the divided portions shift from each other in the lead gripping position, thus causing slip or breakage of the lead.